The fundamental components of an accelerometer device typically comprise a movable mass, one or more mass-supporting flexure elements (flexible arm regions) and a sensing mechanism which provides an (electrical) output representative of the movement of the mass (in response to a force input). In a semiconductor-resident accelerometer the sensing mechanism customarily employs the piezo-resistive properties of one or more relatively thin flexure elements, through which a larger volume, `floating` mass region is joined to a surrounding substrate.
More particularly, as shown in FIG. 1, a typical semiconductor accelerometer structure is formed by selectively etching a semiconductor substrate 10 to define a plurality of reduced thickness arm regions 11 which extend between and adjoin a relatively thick mass region 13 and surrounding material of the substrate 10. In effect, the mass region 13 is supported in a cantilever fashion by the arm regions to the surrounding substrate. By the selective introduction of doping impurities into the arm regions and forming of electrical contacts on the doped regions, a piezo-resistive structure is formed. Because the thickness of supporting arm regions 11 is substantially reduced compared with that of mass region 13, the arms will readily flex in response to a component of force being imparted to the accelerometer structure along a line 14 orthogonal to the substrate surface. As a result of such flexing of the arm regions, there is a change in the magnitude of their piezo-resistance, so that a measurement of acceleration can be derived by output circuitry in which the change in voltage across arm regions 11 is monitored.
Now, while the structure shown in FIG. 1 represents the basic structure of a semiconductor accelerometer, it is only a diagrammatic illustration of the result of an ideal wafer processing sequence, rather than a practical one. Namely, in a practical fabrication process, the parameters of the piezo-resistive regions must be predictable and repeatable. To use a selective etch process to obtain arm regions of precise dimension places such extreme tolerances on the etch process as to render it non-cost effective for commercial production runs.
One approach to solve this precision etch requirement is to use a separate "thin" layer for the accelerometer structure and attach the accelerometer layer to a support substrate in which a recess or cavity in the underlying substrate is provided to accommodate the flexing of the accelerometer arms and movement of the mass region. One example of such a scheme is diagrammatically illustrated in FIG. 2, which shows an underlying support substrate (or handle wafer) 20 having a selectively etched cavity 22 extending from a first surface 24 to some prescribed depth into the substrate. A top semiconductor layer 26 is bonded to support substrate 20 (e.g., means of an oxide layer 28) such that the accelerometer structure, comprised of a mass region 34 and adjoining piezo-resistive arm regions 32, is aligned (directly overlies) with a flex-accommodating cavity 22 in handle wafer 20. The thickness of top layer 26 is then reduced, (chemically or mechanically) and conductivity-modifying impurities are introduced into a pattern of arm regions 32 to form a piezo-resistive semiconductor accelerometer structure in top layer 26. Finally, the top semiconductor layer 26 is selectively etched in accordance with the intended mass/arm pattern to yield an accelerometer topography shown in FIG. 3, wherein a plurality (four, as shown in the plan view of FIG. 4) of arm regions 32 interconnect centrally located region 34 with surrounding material of top semiconductor layer 26. Because the lateral dimensions of central region 34 are greater than those of arm regions 32, region 34 has a greater mass and does not flex to the same degree as arm regions 32, so that region 34 may be considered, in effect, a `mass` region.
Areas 35 of top semiconductor layer 26, adjacent to arm regions 32, are devoid of material as a result of the selective etch, as layer 26 is etched completely through to oxide layer 28. Thereafter, that portion of oxide layer 28 exposed by apertures 35 in top layer 28 is etched by a wet chemical etch, such as buffered HF. The etch is isotropic, so that it removes dielectric (semiconductor oxide) material underlying the relatively narrow arm regions 32, and leaves a wafer bonded accelerometer containing patterned top semiconductor layer 28 overlying flex cavity 22 in substrate 20, as shown in FIG. 4.
Now, although the fabrication techniques described with reference to FIGS. 2-4 allows the dimensions of the accelerometer structure to be precisely defined, it requires that the accelerometer-defining mask overlying bonded layer 28 be precisely aligned with respect to underlying cavity 22, thus complicating and increasing the cost of the process.
For examples of literature illustrating conventional semiconductor accelerometer architectures, such as those described above, attention may be directed to the following articles: "Tiny Accelerometer IC Reaches High Sensitivity" by Richard Nass, Electronic Design, Sep. 22, 1988, pp 170, 171; "Acoustic Accelerometers" by M. E. Motamedi, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. Vol. UFFC-34, No. 2, March 1987, pp 237-242; and "A New Uniaxial Accelerometer in Silicon Based on the Piezojunction Effect" by B. Puers et al, IEEE Transactions on Electron Devices, Vol. 35, No. 6, June 1988, pp 764-770.